1. Field of the Invention
The present invention relates to an accelerometric sensor that can be placed, e.g. as an accelerometer, in missiles or satellites to guide them, or as an inclinometer on an offshore drilling terminal.
More particularly, the sensor works flectionally and comprises a fixed mass part, a mobile mass part and two vibrating beams or blades each having ends rigidly fastened to the mass parts, respectively. Such a sensor takes advantage of the high sensitivity of the frequency of a flectional vibrating beam, under compression or tensile forces that exerted longitudinally to the beam when the mobile mass part is subjected to a transverse acceleration. The signal leaving the sensor is a function of a differential frequency of the two beams which can be easily used to deduct the value of the acceleration or of the banking of a plane by comparison with the earth's gravitational field.
2. State of the Prior Art
Such a sensor called a double resonator cantilever sensor is disclosed in the U.S. Pat. No. 4,479,385 to Koehler. The mass parts are comprised of mass pavings of which one is affixed to a base. The beams are rectangular plates of quartz supporting appropriate electrodes and are completely distinct and separable from the mass pavings. These beams form braces between the mass pavings. Each beam has flat ends respectively fixed to coplanar sides of the pavings.
The known sensor shown in U.S. Pat. No. 4,479,385 offers three main drawbacks.
A first drawback of this known sensor is a low acceleration sensitivity, typically a maximum frequency variation of 100 to 300 Hz for resonator beams with a natural frequency of 40 kHz. The maximum relative frequency variation is therefore in the region of .DELTA.f/f=200/40,000=0.5%.
This drawback is due to the fact that the mobile part is only fastened to the fixed part by two beams with flexural stiffness that is not very high. The mass of the mobile part must therefore be limited to a low value for the natural frequency of the sensor to be greater than 2,000 Hz as per recommendations for accelerometers. Furthermore, the beams may twist due to relatively low stiffness.
Another drawback of the known sensor, also due to the low value of the mobile mass, is in relation to the need for vibratory decoupling of the vibrating beams in order for the latter to vibrate independently of one another. Each of the beams consists preferably of a double-ended tuning fork type resonator.
A third drawback of the known sensor concerns the assembling together of the different elements, mass parts and vibrating beams or transducer beams. This assembly is very significant in the cost price of the sensor, as well as in the sensor performances as a result of its qualities.